1. Field of the Invention
This invention relates to particle separators for use in gas turbine engines.
2. Prior Art
Gas turbine engines have found commercial and military application in a variety of uses. In many of these applications, the engine must operate in an environment laden with dirt and debris. One classic example is the use of gas turbine engines for use in helicopter or VTOL aircraft. These types of aircraft tend to create upon landing or take off an area containing a large volume of entrained dirt and debris. The ingestion of such dirt, sand, or other contaminants into the engine itself may result in considerable damage to the engine in a variety of ways. For example, foreign particles ingested in the engine can rapidly erode the impeller blades, as well as severely damaging compressor elements. When this ingestion occurs, the desired balance condition of the compressor is often distorted and the useful lifetime of the engine is shortened, and in many cases, destoryed. At a minimum, considerable down-time results while engine repairs are effectuated.
Within the prior art, a variety of techniques are known which attempt to reduce the ingestion of contaminants into such engines. A first type of device can be categorized as a barrier type filter, such as an inlet screen or a series of baffles. These inlet screens, or the like, prevent the ingestion of sizeable foreign objects; however, they are not without material shortcomings. Devices of that type may easily become clogged with contaminants and thereby restrict the airflow into the engine such that the large volume of air required for efficient performance is reduced. Hence, while barrier type filters may provide sufficient efficiency in terms of removing dirt, nevertheless, this efficiency is accomplished while compromising engine performance as a result of substantial constriction of intake airflow. Additionally, it has been found that inlet screens of this type can produce a safety hazard when ice forms over the screen itself. Additionally, in some extreme circumstances, the screens themselves break up with disastrous results to the engine.
Another technique known in the prior art to catch sand and entrained dust before entering the turbine engine consists in the installation of an air particle separator comprising approximately 1300 so called strata tubes per engine, each tube having a diameter of approximately 1.5 inches and an overall length of approximately 4 inches. Separators of this known design, while proving relatively successful, are expensive and space consuming. Also, they by nature of their construction result in weight penalties which, in the case of VTOL aircraft, cannot be tolerated.
A second class of devices used for separating entrained debris from gas turbine engines utilizes a rotor structure with veins so configured to provide a high centrifugal force to incoming air, thereby forcing the entrained dirt toward the outer wall of the separator from which point it can be directed to a suitable debris collector. Typical of this art is the patent to Robbins, U.S. Pat. No. 3,302,395. As shown best in FIG. 1 of the Robbins patent, a debris rotor 15 utilizes a helical vein 19 configured at a predetermined orientation such that a high angular velocity is produced in the field of flow as air flows from the inlet through veins 23 to the compressor 5 of the engine. Foreign material such as dirt, sand, water and the like are mixed in the vortex of this swirling air and are, accordingly, centrifuged toward the outer walls of the annular passage 27, and, hence, through the engine bypass ejector duct 29 without ingestion into the compressor sections 5 of the engine. Air which is relatively uncontaminated in suitable amounts for aircraft operation is passed through the center of the engine into the compressor section 5, and, hence, to the combustion section 7 for the production of thrust in the usual manner. Robbins, in FIG. 2, shows an alternative embodiment not utilizing passage through the engine bypass, but, alternatively, utilizes a trap door structure 43 which is used to vent material contained in the debris collection chamber 41 at the extremity of the outer peripheral surface of the stator structure of the compressor 33. Hence, materials which are collected in chamber 45 are conveniently exhausted upon the opening of door 43.
Another variation is shown in the patent to Alsobrooks, U.S. Pat. No. 3,444,672, which discloses in FIG. 2 an air cleaner for turbine engines. Basically, as shown in the figure, air passing through the housing 16 is rotated by means of impeller blades 32 such that the air flow is confined immediately adjacent to the outer periphery of air passage 19. Air bearing a higher concentration of foreign particles will be present immediately adjacent the housing 16, and the cleaner will pass immediately adjacent to the fairing 43 due to the heavier weight of the foreign particles. As a consequence, this cleaner air will be induced into the gas turbine 12 through the discharge passage defined in FIG. 2 as a cylindrical extension 44. In contrast, air bearing a larger percentage of foreign debris or particles is discharged to the atmosphere through a dirt collector scroll 45 formed by the housing 16 adjacent the air outlet 18, which interacts with a flow restrictor passage 47. An outlet duct 48 is provided such that air and the heavier foreign particles separated from the air induced into the gas turbine 12 can be conveniently redistributed into the atmosphere. A similar system is shown in the patent to Roberts, U.S. Pat. No. 3,557,537, and also in Flatt, U.S. Pat. No. 3,616,616.
A slightly modified configuration utilizing centrifugal flow with a helical or spiral vein system is shown in Wilkinson, U.S. Patent No. 3,469,566. In this patent, as shown best in FIG. 3, air enters the system through inlet 18. A concentric central tube 20, having a closed front end 20', is provided on its exterior surfaces with a set of spiral baffles or veins 21. These veins impart a swirling action into the air incoming at 18 such that the large or heavier particles of dust, dirt, etc. are centrifugally extracted from the incoming air, and as a dirty airstream are discharged into chamber 15 at position 19. Relatively clean air flows inwardly into the central tube 20 at location 23 through a series of apertures 23, which are located downstream from the veins 21. This clean air then passes through outlet 24 or utilization in the engine.